Interlock systems are known for securing industrial processes or appliances wherein a safe sequence of operations relating to an industrial process or an appliance is ensured by means of a number of locks. An example of a safety interlock system is a lock which locks an access door to an electrical installation. In order to gain access to the electrical installation itself, the door must be opened. The door can only be opened by unlocking the lock. However, the release movement of the lock simultaneously causes an interruption in the power source of the installation (so-called “power interlocking”) or operates a control circuit with which the power source is disconnected from the electrical installation (“control interlocking”). The interlock system ensures that access is gained to the electrical installation only when it is not in operation and cannot therefore cause any hazard for its environment. In another example of an interlock system, both the switch for switching the power source on and off and the access to the electrical installation are provided with a locking mechanism. With a first key the user can unlock the locking mechanism of the power supply switch and then switch off the power source. The power source is locked in the switched-off state by removing a second key from the locking mechanism, with which second key the locking mechanism of the access can be unlocked. The electrical installation is then accessible. The second key herein remains retained in the access locking mechanism until the access is once again closed and locked. Only when the access in the closed state is unlocked can the second key be removed from the access locking mechanism. The second key is now available to unlock the power supply locking mechanism and switch the power source on again. Depending on the situation and the specific application, there are many possible variants of the above stated safety systems, which can all guarantee an adequate security.
Safety interlock systems are also applied in locking and unlocking closing valves in a chemical plant. A closing valve provided with an interlock system can be locked in the closed position (locked closed system) and/or locked in an opened position (locked open system). By providing closing valves with an interlock, undesired operation thereof can be prevented. It is also possible, using the locking mechanism on a number of different closing valves, to ensure a correct, predetermined sequence of closing or opening of the closing valves, for instance so as to prevent hazardous situations.
The safety system is for instance applied in process installations which are provided with pressure relief systems for safeguarding a pressure vessel. In order to switch off these pressure relief systems for repair or maintenance it is required to isolate these systems by means of blocking valves. If these blocking valves are operated in the wrong sequence, the pressure vessel can become blocked, which can result in a dangerous overpressure. When for instance the pressure relief system of a determined pressure vessel requires maintenance, the pressure vessel must first be connected to an alternative pressure relief system by opening a first valve and locking it in the opened position thereof, while the original pressure relief system can only then be switched off by unlocking a second valve and then closing it. In order to ensure this specific sequence of opening and closing for valves, a user first collects a key from a central control area and unlocks the interlock system of the first valve with this key. The first valve can then be opened, and by locking the first valve in the opened state a second key becomes available which in turn can be removed from the interlock system. The second key is subsequently used to unlock the second valve locked in the opened position, whereafter the second valve can be closed. In this situation the original pressure relief system is switched off, but the alternative pressure relief system is guaranteed to be in operation. No hazardous overpressure situation can hereby occur in the pressure vessel.
Many other types of safety interlock system are known. Some of these are fully integrated with the appliance for controlling, such as for instance a shut-off valve, while other interlock systems are supplied independently of the process or appliance to be controlled, and wherein they can act on the appliance in numerous ways so that a correct control of the appliance is ensured.
The known interlock systems do however have a number of drawbacks. A first drawback is that many different components are required to give the keys and locks unique key and lock codes. This makes the system complex and requires an expensive manufacturing process.
In addition, the different components must be made code-specific relatively early on in the production process. This usually means that the safety systems are already made unique in the factory. A drawback here is that once the known systems have been fitted, any adjustment of or addition to the safety system, when a key is lost or in the case of a possible increase in the number of keys, a separate set of keys must be manufactured in the factory. This means extra transport costs and takes a relatively long time. This is problematic, especially when the system is intended to act on an appliance in a remote location or difficult to reach. When the safety interlock system is for instance applied in safeguarding a process on a drilling platform, it is in most cases difficult or time-consuming to send the keys from the mainland to the platform.
A further drawback of the existing systems is that making the lock and the key unique requires mechanical processing of a number of components of the system, wherein a mechanical process usually entails a number of machining operations. This means that special processing equipment, with which for instance the machining operations on the lock and the key can be performed, must be available at or in the vicinity of the location of use. Such processing equipment is however not normally available on-site.
There is a need to to provide a safety interlock system in which the above stated drawbacks of the prior art, among others, are obviated.